As shown in FIGS. 1A and 1B, pencils are generally made up of a round pencil core 2 surrounded by an outer casing of wood or other material. The outer casing includes an upper section 6 and a lower section 4, with an adhesive layer 8 to bind the upper and lower section. During mass production of pencils, upper 10 and lower 12 slats are used, which have multiple grooves 3a in slat 10 and 3b in slat 12. Pencil cores are placed in the grooves of the lower slat and the upper slat is placed on the lower slat to form the upper and lower section of the pencil casing, respectively. To bond the cores to the slats and the slats to one another, an adhesive is generally used along each channel and at the bonding surface 8 between the two slats. The combination of slats and pencil cores is referred to as a "sandwich."
The bonded slats are then cut into individual pencils in any variety of outer casing styles. These styles are well known and include round and polygonal cross-sections, as shown by the hexagonal outline 14 shown in FIG. 1B.
With colored pencils, because the core is made from waxes, it is difficult to manufacture pencils such that the core remains in the pencil casing and does not rotate. Even with the use of adhesives, color cores often rotate and slide within the pencil casing. Moreover, the methods used to secure the core in the pencil casing should be amenable to mass produced, commercial manufacture of writing instruments.
Previous methods to secure a core to the slat have included both mechanical and chemical means used in conjunction (generally) with an adhesive. Prior mechanical methods of attachment relied on a variety arrangements. For example, as shown in Great Britain Patent 12,446, a series of thin angular ribs or flutes running the length of the core and projecting up from the surface thereof were pressed into wooden slats. The formation of the thin ribs disclosed by this reference is not amenable to modern commercial production methods. Swiss Patent 598,288 discloses a ribbed core held between two plastic by a clamping device. U.S. Pat. No. 551,288 discloses a lower slat having a deep groove for containing an entire pencil core. An upper slat caps off the lower slat and is wedged between the sides of the exposed core to restrain the core in place. In addition, an adhesive is used to restrain the core.
Chemical means, used in conjunction with adhesive, usually involve treatment of the core to allow it to better adhere to the wood slat. These treatments involve the modification of the greasy surface of a color core, to render the surface suitable for bonding with the adhesive. Examples of chemical means are disclosed in U.S. Pat. Nos. 1,892,508, 1,958,194, and 2,116,217. None of these previous methods have been entirely successful.
Thus, there is a need to design a cost efficient pencil having a core that is joined to a pencil slat in a manner that the core does not become displaced and does not rotate within the slat, and without the need of an adhesive.